Optical data storage systems are of great commercial and academic interest because of their potential for very high density data storage. Unlike magnetic recording where data density may be limited by particle size, the density of optical recording is limited only by the diffraction limit of the illuminating light. In practice, the data density is in part also limited by how small illuminating radiation, such as a laser beam, can be focussed on the disk. To reduce the laser spot diameter, several methods can be employed. Higher frequency light may be used because it has a shorter wavelength. Increasing the numerical aperture of the lens may decrease spot size. SIL system, incoming converging rays from an objective lens are refracted at a spherical surface of the SIL, resulting in an increased effective numerical aperture. More details on the effect of a SIL are described in U.S. patent application Ser. No. 08/641,513, filed May 1, 1996, and Ser. No. 08/657,145, filed Jun. 3, 1996, both assigned to the assignee of the present invention, and both of which are hereby incorporated by reference.
An object of the invention is to provide a multilayer optical recording system in which a SIL is used in a multilayer optical recording system for optimum data densities.